The present invention relates to a Peltier module which is an electronic temperature controller used for maintaining constant a temperature of a semiconductor laser chip and an optical system, a Peltier module integrated heat spreader, and a laser module including the Peltier module as a temperature controller.
In general, laser diode modules have been used as a signal light source of optical fiber communication, particularly trunk line system and CATV or a pump light source of a fiber amplifier. In such laser diode modules, in order to implement high output and stable operation, a Peltier module is incorporated and optical parts such as a laser diode chip, a photodiode chip or a lens and electrical parts such as a thermistor element, an inductor or a resistor are placed on a substrate which is arranged on the Peltier module.
In general, when two kinds of conductors A, B are connected and a current is passed thereto under the condition of a constant temperature, heat generation or heat absorption occurs in a point of contact between the conductors A and B. This is called a Peltier effect. A Peltier module uses this principle. A conventional Peltier module is shown in FIG. 4. As shown in FIG. 4, a p-type semiconductor element 101 and an n-type semiconductor element 102 which are thermoelectric elements are alternately arranged in parallel and electrodes 103, 104 are placed in both ends of respective semiconductor elements. Both ends of respective semiconductor elements are joined to the electrodes by solder. The p-type semiconductor element 101 and the n-type semiconductor element 102 are joined in series electrically through the electrodes alternately.
Further, in order to electrically insulate an electric circuit formed by the electrodes 103, 104, the p-type semiconductor element 101 and the n-type semiconductor element 102 which are the thermoelectric elements from the outside, a pair of electrical insulating substrates 105, 106 are provided in the respective outsides (namely, upper and lower sides in the figure) of the electrodes 103, 104, and the electrodes are joined to the electrical insulating substrates by solder. Thus, the Peltier module has a structure in which the electric circuit formed by the electrodes, the p-type semiconductor element and the n-type semiconductor element is sandwiched by two electrical insulating substrates. As the electrical insulating substrates described above, ceramics are commonly used. According to the Peltier module described above, the heat is moved from the electrical insulating substrate 105 to the electrical insulating substrate 106, thus the electrical insulating substrate 105 is cooled.
In a laser diode module, it is important to cause a semiconductor laser chip to stably oscillate which is a light source, and further prevent deterioration of performance due to a heat. For this purpose, the semiconductor laser chip is connected to the Peltier module through a mount so as to keep constant a temperature of the semiconductor laser chip. In particular, in a pump laser used for signal amplification as a part of an optical fiber communication system, accurate temperature control is required since stability of the wavelength and output thereof is required.
In a pump laser used for an optical signal amplifier, alumina (Al2O3) which is ceramic material is commonly used in a substrate of the Peltier module (a metal plating layer or a metal thin film layer is formed in advance on this ceramic substrate). A lower substrate of the Peltier module and a bottom plate of the laser module are joined by solder and further, an upper substrate of the Peltier module and a semiconductor laser mount is also joined by solder.
Since the alumina (Al2O3) used as an insulating substrate of the Peltier module has small thermal conductivity, the heat which is conducted from the semiconductor laser chip to the alumina substrate through the mount is not sufficiently conducted to the periphery of the substrate. As a result, the temperature is high in the central portion of the alumina substrate whereas the temperature is low in the end portion of the alumina substrate, thus the temperature of the substrate is not uniform.
When the Peltier module is driven under the condition in which the temperature of the substrate is not uniform, the heat is transferred in the central portion of the Peltier module, however the amount of the heat transfer becomes small in the edge portion of the Peltier module. As a result, there is a problem in which the amount of the transferred heat in the Peltier module as a whole becomes small and the transferred heat corresponding to driving power of the Peltier module, namely heat transfer efficiency, is lowered. Therefore, a larger current energy is consumed to transfer the heat generated from the semiconductor laser chip.
In order to solve the above-mentioned problem, there is also proposed a technique for reducing temperature differences in the substrate of a Peltier module by bringing the whole surface of the substrate of the Peltier module into contact with the laser diode chip mount in which a shape of the bottom of the mount is formed into the same shape as those of the substrate of the Peltier module or the size of the bottom of the mount is formed to be larger than the size of the substrate of the Peltier module.
However, in case that warp deformation (i.e, bending) occurs in the substrate of the Peltier module due to heating and cooling, the warp deformation extends to a plate of the mount, and a risk of causing a critical defect in which a deviation (i.e., misalignment) occurs in an optical system comprising a semiconductor laser, a lens and an optical fiber becomes larger. Since the optical system of such a laser module is adjusted very accurately, an output of the laser module is remarkably reduced even in case that the misalignment slightly occurs in the optical system.
Further, when alumina (Al2O3) with small thermal conductivity is used as an insulating substrate of the Peltier module, a temperature gradient in a depth direction (that is, plate thickness direction) of the substrate becomes large and as a result, a temperature difference becomes large between the joint portion of a high temperature side and the joint portion of a low temperature side of a thermoelectric element (circuit) at the time of driving the Peltier module. In order to transfer a constant amount of heat in a state in which the temperature difference is large between the joint portion of the high temperature side and the joint portion of the low temperature side of the thermoelectric element (circuit), thus, electric power necessary to drive the Peltier module becomes larger and as a result, a problem occurs that the amount of heat transfer with respect to driving power of the Peltier module, namely heat transfer efficiency is lowered.
Further, it is also proposed that aluminum nitride (AlN) with high thermal conductivity be used as the insulating substrate of the Peltier module. The conventional problem that the heat transfer efficiency is lowered when the alumina (Al2O3) is used in the substrate of the Peltier module can be solved by using the aluminum nitride (AlN) in the substrate of the Peltier module.
However, the aluminum nitride (AlN) differs in a thermal expansion coefficient from CuW used for a base plate (that is, case bottom plate) as well as a mount of a laser module. In addition, the insulating substrate of the Peltier module differs in the amount of expansion or shrinkage from the case bottom plate or the mount in the case of expansion or shrinkage by heating or cooling at the time of driving the laser module. As a result, there is a problem that thermal stress applied to a joint solder face becomes large.
Therefore, when the thermal stress is repeatedly applied to a solder joint face, there is a problem that creep or peeling occurs on the joint face. Further, since the aluminum nitride (AlN) has properties that mechanical strength is weaker as compared with the alumina (Al2O3), there is a problem in which a risk of the damage in the insulating substrate itself of the Peltier module increases more when expansion or shrinkage occurs in the insulating substrate of the Peltier module by heating or cooling at the time of driving the laser module.
Further, in the laser module, higher output and smaller power consumption have been required. Thus, it is also necessary to reduce power consumption itself of the Peltier module.
Therefore, it is desired to provide a Peltier module and a laser module in which a large variations in temperature do not occur in the central portion and the end portion of the substrate of the Peltier module, thus the heat transfer efficiency is not lowered and the substrate of the Peltier module is not damaged and power consumption is small and reliability is high.
One embodiment of a Peltier module of the invention is a Peltier module comprising thermoelectric elements in which plural p-type and n-type thermoelectric elements are alternately arranged, metal electrodes placed in both ends of the thermoelectric elements in order to connect the thermoelectric elements in series, and metal substrates on at least a part of which surfaces an insulating thin film is formed, the metal substrates being oppositely placed so as to be connected to the metal electrodes and sandwich the metal electrodes and the thermoelectric elements.
One embodiment of a laser module of the invention is a laser module comprising (1) a laser diode (LD) element, (2) a photo coupling member for coupling laser light from the LD element to an optical fiber, and (3) a temperature control member for stabilizing an oscillation state of the LD element, the temperature control member including a Peltier module comprising thermoelectric elements in which plural p-type and n-type thermoelectric elements are alternately arranged, metal electrodes placed in both ends of the thermoelectric elements in order to connect the thermoelectric elements in series, and metal substrates on at least a part of which surfaces an insulating thin film is formed, the metal substrates being oppositely placed so as to be connected to the metal electrodes and sandwich the metal electrodes and the thermoelectric elements.
One embodiment of a Peltier module integrated heat spreader of the invention is a Peltier module integrated heat spreader including a Peltier module comprising thermoelectric elements in which plural p-type and n-type thermoelectric elements are alternately arranged, metal electrodes placed in both ends of the thermoelectric elements in order to connect the thermoelectric elements in series, and two metal substrates on at least a part of which surfaces an insulating thin film is formed, the metal substrates being oppositely placed so as to be connected to the metal electrodes and sandwich the metal electrodes and the thermoelectric elements, wherein one of two metal substrates is connected to a heat generating element and the remaining metal substrate is extended to form a leg portion, said leg portion covering the heat generating element mounted on an electronic board.